Regarding a method for molding a fiber-reinforced thermoplastic plastic, stamping molding, by which a laminated base material that is obtainable by laminating a prepreg produced by impregnating continuous reinforcement fibers with a thermoplastic resin, is preheated or heated and pressurized with a press machine or the like, and thereby the laminated base material is shaped into a desired shape, is most commonly carried out. A fiber-reinforced plastic obtained thereby has excellent mechanical properties because continuous reinforcement fibers are used. Furthermore, a fiber-reinforced plastic can be designed to have required mechanical properties, by aligning the continuous reinforcement fibers in a regular fashion. However, since the reinforcement fibers are continuous reinforcement fibers, it is difficult to form a complicated shape such as a three-dimensional shape with the continuous reinforcement fibers, and the shape is mostly limited to members having shapes close to a planar shape.
In order to solve this problem, there have been disclosed a laminated base material having excellent stamping moldability, which is obtainable by dispersing, on a plane, chopped prepregs that are produced by cutting tape-like prepregs having a narrow width to a constant length; and a laminated base material which is considered to exhibit excellent shapeability at the time of molding and excellent mechanical properties after shaping, the laminated base material being obtainable by inserting incision into a prepreg formed from continuous fibers and a thermoplastic resin (for example, Patent Documents 1 and 2).
However, the laminated base materials described in Patent Documents 1 and 2 have a problem that at the time of conveyance after a preheating process during stamping molding, heat sagging (that is, draw-down) of the laminated base material occurs, which is attributable to the fact that the reinforcement fibers are non-continuous, and this heat sagging causes shape change or acceleration of the cooling rate.
As described above, from the viewpoint that a laminated base material undergoes draw-down at the time of preheating or heating, there has been suggested a laminated base material in which a skin material having a high softening temperature is provided in the skin layer of the laminated base material, while a conventionally known fiber-reinforced layer is provided in the core layer (for example, Patent Document 3). However, when a resin layer having a high softening temperature is provided as the skin material, there is a problem that although draw-down at the time of preheating can be suppressed, the skin material inhibits the flow of the reinforcement fibers and the resin at the time of stamping molding, and molding of a three-dimensional shape such as a rib or a boss is significantly deteriorated.